Exemplary conventional anti-lock brake systems (ABS systems) are described in, e.g., Bosch Technische Berichte (Bosch Technical Reports) Vol. 7 (1980), No. 2. In such anti-lock control systems, the brake systems of the vehicle wheels are controlled in such a way that brake pressure is modified as a function of an instability value. Thus, this instability value is generated as a function of the wheel movement that has been detected, generally the wheel rpm. In particular, this instability value is a combination of the instantaneous circumferential deceleration or acceleration of the wheel and the brake slippage. Modification of an instability value of this kind is described in German Patent Application No. 196 11 491.
As described in German Patent Application No. 195 16 120 (corresponding to U.S. Pat. No. 5,556,174), in the case of front-wheel-drive vehicles it is known that vibrations occur at the driven wheels during ABS control procedures, these being caused by flexibility between the engine and the wheels. In order to detect these vibrations, the speed at the drive axle differential is determined. It is then determined whether this speed has superimposed thereon a signal having a frequency which lies within a range defined by the drive train vibrations. In response to the drive train vibrations detected, the activation times of the brake pressure control devices are modified so that the drive train vibrations decay.
Furthermore, vehicles in which the wheels on the rear axle as well as those on the front axle are driven are known. As a general rule, in four-wheel-drive vehicles of this kind, the differential/differential gear unit can be locked, at least partially. In four-wheel-drive vehicles, axle vibrations may occur during ABS braking, particularly if a differential/differential gear unit is at least partially locked.
One of the objects of the present invention is to dampen the vibrations in the drive train in four-wheel-drive vehicles in an effective manner.
The present invention relates to a method and device for controlling the braking action of at least one wheel brake of a four-wheel-drive motor vehicle having at least one rear axle and one front axle. Thus, a first rpm signal representing the speeds of rotation of the front-axle wheels and a second rpm signal representing the speeds of rotation of the rear-axle wheels are detected. Next, the difference between the first and second rpm signals is determined and braking action is controlled as a function of the difference that has been determined.
In particular, a quantity representing the characteristics of the road, and in particular, the coefficient of friction of the road, is determined, and braking action is subjected to control based on the difference that has been determined if the quantity is less than a threshold value. The braking action may only be subjected to control as a function of the difference that has been determined if the quantity that has been determined is less than a threshold value.
In one embodiment of the present invention, particularly in the drive train, the motor vehicle has a differential/differential gear unit that can be at least partially locked. The braking action is subjected to control according to the present invention as a function of the difference that has been determined if the differential/differential gear unit is at least partially locked.
As described above, in four-wheel-drive motor vehicles having a lockable differential, the coupling between the drive train and the driven wheels can cause axle vibrations during an anti-lock control procedure, in particular due to stress acting on the wheels. In particular, this occurs on road surfaces having a relatively low coefficient of friction (e.g., in ice or snow). These axle vibrations may impair the efficacy of the anti-lock control system (ABS), which is intended to diminish the wheels"" tendency to lock by braking interventions. They may also impair driving and braking comfort during braking procedures on road surfaces having a low or medium coefficient of friction (i.e., in ice or snow).
As part of an ABS control procedure, it is preferable to determine a reference speed that represents the vehicle""s forward speed. As a general rule, this is based on the wheel rpm rates that have been detected. Axle vibrations having a high frequency and the resulting resonances may disrupt detection of the reference speed and thus considerably disrupt the entire ABS control procedure. According to the present invention, the level of axle vibrations is measured by determining, for each axle, the difference
DVaxle=(Vxe2x80x94VL+Vxe2x80x94VR)xe2x88x92(Vxe2x80x94HL+Vxe2x80x94HR)
between wheel speeds (V_VL, V_VR, V_HL, V_HR).
The measure designed to counteract axle vibrations according to the present invention prevents and damps the vibrations in an effective manner, and is therefore suitable for preventing these vibrations or reducing their level, so that ABS control remains fully effective.
It is also advantageous to subject the braking action to control according to the present invention as a function of difference DVaxle that has been determined if difference DVaxle that has been determined is less than and/or greater than at least one threshold value. The braking action may only be subjected to control as a function of the difference that has been determined if difference DVaxle that has been determined is less than and/or greater than at least one threshold value.
In the case of hydraulic brake systems, the braking action is controlled by feeding/discharging brake fluid to/from the wheel brakes. As a general rule, controllable intake/discharge valves are provided for this purpose. As soon as the axle vibrations have been measured via quantity DVaxle, these axle vibrations are prevented/damped/reduced according to the present invention by generating suitable activation values for the brake fluid intake/discharge valves, so that quantity DVaxle is reduced as soon as it exceeds a specific positive threshold value or falls below a specific negative threshold value. There are two options for this:
According to a first option, braking action is controlled as a function of determined difference DVaxle in such a way that braking action is increased on the vehicle axle having the higher rpm rates.
According to a second option, braking action is controlled as a function of determined difference DVaxle in such a way that braking action is reduced on the vehicle axle having the lower rpm rates.
As described above, in the ABS systems, the braking action is generally modified as a function of an instability value. An activation of the intake/discharge valves for the brake fluid is based on this instability value. To allow the braking action in the ABS system to be subjected to control (e.g., an increase or reduction of the braking pressure) as a function of vibration value DVaxle according to the present invention, the instability value is modified as a function of value DVaxle.
One of the objects of the present invention is to reduce the axle vibrations, represented by value DVaxle, and/or keep them as low as possible so that no difficulties will be expected with the ABS control procedure. For safety reasons, preferably only the braking action and/or the brake pressure are subjected to the reduction described above as a function of DVaxle.